The data from recent experiments in my laboratory strongly suggest that the inferior colliculus (IC) of Mexican free-tailed bats is functionally divided into two major subdivisions; a medial division specialized for processing echolocation signals and a lateral region specialized for processing communication sounds. The features which distinguish the two regions are marked differences in tonotopic organization as well as in tuning and discharge properties. In order to explore the possible role of the lateral region of the IC in the encoding of communication signals, I am proposing a series of behavioral, neurophysiological, and neuroanatomical experiments. The behavioral experiments are designed to record and catalogue at least some of the communication sounds used by Mexican free-tailed bats and to determine the social context in which the sounds are used. The neurophysiological experiments will examine the neuronal response features of individual neurons evoked by electronically generated sounds having power spectra similar to those of the natural communication sounds. Furthermore, I intend to evaluate and compare how neurons in the lateral (communication) and medical (echolocation) regions respond to natural communication sounds that are previously recorded and then played back on tape loops. Finally, the neuroanatomical studies will utilize the retrograde tracer, horseradish peroxidase, to trace the inputs which project to the lateral (communication) and medial (echolocation) regions of the IC. With this technique, I hope to determine whether the medial and lateral regions of the IC have inputs from common brainstem nuclei or whether there are separate lower centers that project differentially upon each.